US7567625B2 - Apparatus and method for sub-carrier allocation in a multiple-input and multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) communication system - Google Patents

Apparatus and method for sub-carrier allocation in a multiple-input and multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) communication system Download PDF

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US7567625B2
US7567625B2 US10/984,802 US98480204A US7567625B2 US 7567625 B2 US7567625 B2 US 7567625B2 US 98480204 A US98480204 A US 98480204A US 7567625 B2 US7567625 B2 US 7567625B2
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sub
carriers
allocated
user equipment
user
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US20050099937A1 (en
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Hyun-Seok Oh
Hyun-Seok Yu
Hye-Jeong Lee
Gin-Kyu Choi
Yong-Suk Moon
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Samsung Electronics Co Ltd
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J11/00—Orthogonal multiplex systems, e.g. using WALSH codes
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00—Arrangements affording multiple use of the transmission path
    • H04L5/003—Arrangements for allocating sub-channels of the transmission path
    • H04L5/0037—Inter-user or inter-terminal allocation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/02—Arrangements for detecting or preventing errors in the information received by diversity reception
    • H04L1/06—Arrangements for detecting or preventing errors in the information received by diversity reception using space diversity
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00—Arrangements affording multiple use of the transmission path
    • H04L5/003—Arrangements for allocating sub-channels of the transmission path
    • H04L5/0044—Arrangements for allocating sub-channels of the transmission path allocation of payload
    • H04L5/0046—Determination of how many bits are transmitted on different sub-channels
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00—Arrangements affording multiple use of the transmission path
    • H04L5/003—Arrangements for allocating sub-channels of the transmission path
    • H04L5/0058—Allocation criteria
    • H04L5/006—Quality of the received signal, e.g. BER, SNR, water filling
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00—Arrangements affording multiple use of the transmission path
    • H04L5/0001—Arrangements for dividing the transmission path
    • H04L5/0014—Three-dimensional division
    • H04L5/0023—Time-frequency-space

Definitions

  • the present invention relates to a mobile communication system. More particularly, the present invention relates to an apparatus and a method for allocating sub-carriers in a multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) mobile communication system.
  • MIMO multi-input multi-output
  • OFDM orthogonal frequency division multiplexing
  • IMT-2000 International Mobile Telecommunication-2000
  • the 3G mobile communication system is evolving into a 4G mobile communication system.
  • the 4G mobile communication system may provide integrated wired/wireless communication services by effectively combining a wireless communication network with a wired communication network.
  • techniques for transmitting high-speed data services in the 4G mobile communication system are now being standardized.
  • the transmitted signal is subject to multipath interference due to various obstacles existing between a transmitter and a receiver.
  • Characteristics of the wireless channel having multipaths are determined by a maximum delay spread and a transmission period of a signal. If the transmission period of the signal is longer than the maximum delay spread, interference may not occur between continuous signals and a frequency characteristic of a channel is determined as frequency nonselective fading.
  • the OFDM scheme is a type of a multi carrier modulation (MCM) scheme, in which serial symbol arrays are converted into parallel symbol arrays, which are modulated into a plurality of sub-carriers, that is, a plurality of sub-carrier channels which are orthogonal to each other.
  • MCM multi carrier modulation
  • the OFDM scheme has been widely applied to digital transmission techniques such as digital audio broadcasting (DAB), wireless local area network (WLAN), and wireless asynchronous transfer mode (WATM). That is, the OFDM scheme, which is rarely used due to its complexity of hardware, can be adopted in the actual system as digital signal processing techniques, such as Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT), have been developed.
  • DAB digital audio broadcasting
  • WLAN wireless local area network
  • WATM wireless asynchronous transfer mode
  • FFT Fast Fourier Transform
  • IFFT Inverse Fast Fourier Transform
  • the OFDM scheme is similar to an Frequency Division Multiplexing (FDM) scheme.
  • the OFDM scheme transmits a plurality of sub-carriers while maintaining orthogonality therebetween, thereby obtaining an optimum transmission efficiency when transmitting high-speed data.
  • the OFDM scheme since the OFDM scheme has superior frequency utilization and represents superior endurance against multipath fading, it may obtain optimum transmission efficiency when transmitting high-speed data.
  • the OFDM scheme since the OFDM scheme uses overlapping frequency spectrums, it can obtain superior frequency utilization. Furthermore, the OFDM scheme represents superior resistance against frequency selective fading and multipath fading, reduces an affect of Intersymbol interference (ISI) by using a guard interval, simplifies a structure of an equalizer, and reduces impulse-type noise. Thus, the OFDM scheme is positively utilized in a communication system.
  • ISI Intersymbol interference
  • FIG. 1 is a view illustrating a transmitter of a conventional OFDM mobile communication system.
  • the OFDM mobile communication system includes a transmitter 100 and a receiver 150 .
  • the transmitter 100 includes an encoder 104 , a symbol mapper 106 , a serial to parallel converter 108 , a pilot symbol inserter 110 , an IFFT unit 112 , a parallel to serial converter 114 , a guard interval inserter 116 , a digital to analog converter 118 , and a radio frequency (RF) processor 120 .
  • RF radio frequency
  • user data 102 with user data bits and control data bits are output to the encoder 104 .
  • the encoder 104 codes user data 102 through a predetermined coding scheme and sends the data to the symbol mapper 106 .
  • the encoder 104 may code the user data 102 through a turbo coding scheme or a convolution coding scheme having a predetermined coding rate.
  • the symbol mapper 106 modulates coded bits through a predetermined modulation scheme, thereby generating modulated symbols and sending the modulated symbols to the serial to parallel converter 108 .
  • the predetermined modulation scheme includes a binary phase shift keying (BPSK) scheme, a quadrature phase shift keying (QPSK) scheme, a quadrature amplitude modulation (16 QAM) scheme, or a quadrature amplitude modulation (64 QAM) scheme.
  • BPSK binary phase shift keying
  • QPSK quadrature phase shift keying
  • 16 QAM quadrature amplitude modulation
  • 64 QAM quadrature amplitude modulation
  • the serial to parallel converter 108 Upon receiving the serial modulated symbols from the symbol mapper 106 , the serial to parallel converter 108 converts the serial modulated symbols into parallel modulated symbols and sends the parallel modulated symbols to the pilot symbol inserter 110 .
  • the pilot symbol inserter 110 inserts pilot symbols into the parallel modulated symbols and sends the parallel modulated symbols having the pilot symbols to the IFFT unit 112 .
  • the IFFT unit 112 Upon receiving signals from the pilot symbol inserter 110 , the IFFT unit 112 performs N-point IFFT with respect to the signals and sends the signals to the parallel to serial converter 114 .
  • the parallel to serial converter 114 Upon receiving the signals from the IFFT unit 112 , the parallel to serial converter 114 converts the signals into serial signals and sends the serial signals to the guard interval inserter 116 .
  • the guard interval inserter 116 which has received the serial signals from the parallel to serial converter 114 , inserts guard interval signals into the serial signals and sends the signals to the digital to analog converter 118 . Insertion of the guard interval is necessary to remove interference between an OFDM symbol transmitted during a previous symbol transmission duration and an OFDM symbol to be transmitted in the present OFDM symbol transmission duration when OFDM signals are transmitted from an OFDM communication system. Such a guard interval has been suggested in such a manner that null data are inserted in the guard interval with a predetermined interval.
  • cyclic prefix in which predetermined last bits of an OFDM symbol in a time domain are copied and inserted into an effective OFDM symbol
  • cyclic postfix in which predetermined first bits of an OFDM symbol in a time domain are copied and inserted into an effective OFDM symbol
  • the digital to analog converter 118 Upon receiving signals from the guard interval inserter 116 , the digital to analog converter 118 converts the signal into an analog signal and sends the analog signal to the RF processor 120 .
  • the RF processor 131 includes a filter and a front end unit. The RF processor 131 transmits the signal output from the digital to analog converter 118 to air through a Tx antenna after RF-processing the signal.
  • the structure of the receiver 150 is reverse to the structure of the transmitter 100 .
  • the receiver 150 includes an RF processor 152 , an analog to digital converter 154 , a guard interval remover 156 , a serial to parallel converter 158 , an IFFT unit 160 , a pilot symbol extractor 162 , a channel estimator 164 , an equalizer 166 , a parallel to serial converter 168 , a symbol demapper 170 , a decoder 172 and a data receiver 174 .
  • noise is added to the signal transmitted from the transmitter 100 while the signal is being passed through a multipath channel. Then, the signal is transmitted to the receiver 150 through a reception antenna. The signal received through the reception antenna is input into the RF processor 152 . The RF processor 152 down-converts the signal received through the reception signal such that the signal has an intermediate frequency band and sends the signal to the analog to digital converter 154 . The analog to digital converter 154 converts the analog signal of the RF processor 152 into a digital signal and sends the digital to the guard interval remover 156 .
  • the guard interval remover 156 Upon receiving the digital signal from the analog to digital converter 154 , the guard interval remover 156 removes the guard interval signals and sends serial signals to the serial to parallel converter 158 .
  • the serial to parallel converter 158 which received the serial signals from the guard interval remover 156 , converts the serial signals into parallel signals and sends the parallel signals to the IFFT unit 160 .
  • the IFFT unit 160 performs an N-point IFFT with respect to the parallel signals output from the guard interval remover 156 and sends the signals to the equalizer 166 and the pilot symbol extractor 162 .
  • the equalizer 166 Upon receiving the signals from the FFT unit 160 , the equalizer 166 performs channel equalization with respect to the signals and sends the signals to the parallel to serial converter 168 .
  • the parallel to serial converter 168 converts the parallel signals into serial signals and sends the serial signals to the symbol demapper 170 .
  • the signal output from the IFFT unit 160 is input into the pilot symbol extractor 162 so that the pilot symbol extractor 162 detects pilot symbols from the signals of the IFFT unit 160 .
  • the pilot symbols detected by the pilot symbol extractor 162 are sent to the channel estimator 164 .
  • the channel estimator 164 performs channel estimation by using the pilot symbols output from the pilot symbol extractor 162 and sends the channel estimation result to the equalizer 166 .
  • the receiver 150 generates channel quality information (CQI) corresponding to the channel estimation result and sends the CQI to the transmitter 100 through a CQI transmitter (not shown).
  • CQI channel quality information
  • the symbol demapper 170 demodulates the signals output from the parallel to serial converter 168 through a predetermined demodulation scheme and sends the decoded signals to the decoder 172 .
  • the decoder 172 Upon receiving the demodulated signal from the symbol demapper 170 , the decoder 172 decodes the demodulated signals through a predetermined decoding scheme, and then, outputs the demodulated signals.
  • the demodulation and decoding schemes employed in the receiver 150 correspond to the modulation and coding schemes employed in the transmitter 100 .
  • the OFDM scheme has been actively studied as a representative communication scheme for a 4G mobile communication system and a next generation communication system.
  • the OFDM uses a plurality of sub-carriers having orthogonality in order to improve frequency utilization, employs Inverse Fast Fourier Transform and Fast Fourier Transform (FFT) so as to easily process high-speed data, and utilizes a “cyclic prefix” in order to improve endurance against multi-path fading.
  • FFT Inverse Fast Fourier Transform and Fast Fourier Transform
  • cyclic prefix in order to improve endurance against multi-path fading.
  • the OFDM scheme is applicable for a multiple-input and multiple-output (MIMO) system.
  • MIMO multiple-input and multiple-output
  • the OFDM scheme used for a single user is different from the OFDM scheme used for a plurality of users.
  • the OFDM scheme used for a plurality of users must allocate sub-carriers such that the sub-carriers do not overlap each other while taking the transmission rate and transmission power per each user into consideration.
  • various sub-carrier allocation methods have been suggested for the OFDM schemes.
  • One of the methods is to minimize total transmission power while using a bit rate for each user as a constraint by using an algorithm based on a Lagrange optimization method.
  • the Lagrange optimization method can attain an optimal solution, it is complex and has a low convergence speed.
  • a two-step sub-carrier allocation scheme in which the number of sub-carriers and transmission power to be allocated to each user are determined and the sub-carriers are allocated to each user such that a maximum data transmission rate can be obtained.
  • a Hungarian algorithm is used for the sub-carrier allocation, resulting in complexity even if the number of users is small.
  • a water-filling algorithm is used for multi-users.
  • a transmit diversity scheme using multiple antennas includes an open-loop scheme and a closed-loop scheme.
  • the open-loop scheme has no feedback information and includes a space-time scheme including a space-time block code (STBC), a space-time trellis code (STTC) and a layered space-time code.
  • the closed-loop scheme has feedback information and includes a transmit antenna array scheme in which each antenna transmits sub-carriers while adding a weight to sub-carriers by calculating the weight based on channel information of the signal transmitted through the antenna.
  • An antenna selective transmit diversity scheme can be utilized in relation to the open/closed loop schemes.
  • the same number of sub-carriers is allocated to each antenna. For instance, if two antennas are provided, odd sub-carriers are allocated to the first antenna and even sub-carriers are allocated to the second antenna.
  • the sub-carrier is allocated to an antenna having a superior channel characteristic by using channel information of the antennas, so the same number or a different number of sub-carriers is allocated to each channel.
  • ADC analog to digital converter
  • DAC digital to analog converter
  • a coding scheme and a clipping scheme are proposed in order to reduce the PARR.
  • a code capable of reducing the PARR in relation to a plurality of sub-carriers but also interference between an in-band frequency and an out-of-band frequency may increase.
  • multiple antennas can be used to reduce the PARR.
  • the PARR can be reduced in a ratio of 10 log N/n with respect to N sub-carriers, herein N is a number of sub-carriers and n is a number of antennas. However, it is necessary to effectively adjust the number of sub-carriers when the sub-carriers are allocated to each antenna.
  • transmission power must be considered in the OFDM scheme in addition to the PARR.
  • power of the sub-carrier allocated to each user must be properly adjusted in order to reduce interference between users. Since the open-loop scheme of the transmit diversity method does not receive feedback information from a receiving terminal, an overhead may not occur. However, it is necessary to transmit the sub-carrier to an antenna even in an inferior channel state of the antenna, so performance degradation may occur.
  • the closed-loop scheme allocates the sub-carrier to each antenna based on a channel state of the antenna so the closed-loop scheme represents superior performance. However, the closed-loop scheme may induce overhead due to feedback information thereof, so it is necessary to reduce the overhead.
  • the closed-loop scheme optimizes transmission power by using a variety of channel estimation information, so a complex algorithm is required.
  • the Lagrange optimization algorithm represents complexity which is almost impossible to realize in an actual system.
  • various simplified sub-optimal algorithms have been suggested, but unnecessarily require overlapping loops and a plurality of sorting operations.
  • an object of the present invention is to provide an apparatus and a method capable of effectively allocating sub-carriers for a plurality of users and transmitting the sub-carriers to users through multiple antennas in an orthogonal frequency division multiplexing (OFDMA) mobile communication system.
  • OFDMA orthogonal frequency division multiplexing
  • an apparatus and method for allocating a plurality of sub-carriers to at least one user equipment and mapping the allocated sub-carriers with one of a plurality of antennas in a multi-input multi-output (MIMO) OFDMA mobile communication system comprise receiving channel information related to the sub-carriers regarding the antennas from a plurality of user equipments; determining a number of sub-carriers to be allocated to each user equipment based on the channel information; and allocating sub-carriers to each user equipment and transmit antennas according to the channel information and the number of sub-carriers.
  • MIMO multi-input multi-output
  • FIG. 1 is a block diagram illustrating a structure of a transceiver of a conventional orthogonal frequency division multiplexing (OFDM) system
  • FIG. 2 is a block diagram illustrating a structure of a multi-input multi-output (MIMO) OFDMA system according to a first embodiment of the present invention
  • FIG. 3 is a flow chart illustrating a sub-carrier allocation procedure in a MIMO OFDM scheme according to a first embodiment of the present invention
  • FIG. 4 is a flow chart illustrating a procedure for determining the number of sub-carriers to be allocated to each user according to a first embodiment of the present invention
  • FIG. 5 is a flow chart illustrating a procedure for determining an antenna and a sub-carrier for each user according to a first embodiment of the present invention
  • FIG. 6 is a block diagram illustrating a structure of a MIMO OFDM system according to a second embodiment of the present invention.
  • FIG. 7 is a flow chart illustrating a procedure for allocating a sub-carrier for each user according to a second embodiment of the present invention.
  • FIG. 8 is a graph illustrating a test result of the present invention.
  • the embodiments of the present invention provide an algorithm for effectively allocating a sub-carrier to each antenna and each user in a multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) system.
  • MIMO multi-input multi-output
  • OFDM orthogonal frequency division multiplexing
  • a system for allocating a sub-carrier according to the present invention can reduce transmission power and peak to average power ratio (PARR), satisfying a user's demand for quality of service).
  • PARR peak to average power ratio
  • the OFDMA scheme used which is a type of OFDM schemes for a plurality of users, must allocate different sub-carriers for each user.
  • one sub-carrier can be determined as an optimum sub-carrier for a plurality of users.
  • an embodiment of the present invention provides a MIMO OFDMA system capable of effectively allocating the sub-carriers depending on a channel environment of an antenna, thereby reducing PARR and improving frequency utilization.
  • channel information of each sub-carrier transmitted through the antennas is required.
  • the channel information is calculated at a receiving terminal and sent to a transmission terminal through a feedback channel. If a large number of sub-carriers are transmitted to the transmission terminal, calculation for the feedback information, reliability for the feedback information, and delay of a feedback path must be considered.
  • a sub-carrier allocation algorithm in an adaptive sub-carrier allocation (ASA) scheme of a closed-loop transmit diversity is provided in an embodiment of the present invention.
  • ASA adaptive sub-carrier allocation
  • channel information for all sub-carriers in antennas is obtained before a user signal is transmitted. Then, the user signal is transmitted by using the channel information. In addition, the channel information is fed back after a predetermined time lapse for the sake of channel state variation. At this time an amount of the channel information is M ⁇ N, in which M is the number of antennas, and N is the number of sub-carriers.
  • the number n k of sub-carriers to be allocated to each user in OFDMA scheme is determined. Then, sub-carriers to be allocated to each user are determined through a bit loading algorithm.
  • the OFDMA scheme is applied to the MIMO system, the sub-carriers to be allocated to each antenna are determined through the bit loading algorithm.
  • the number of sub-carriers and transmission power to be allocated to each user may be determined by taking a minimum bit rate B min (k) required by each user and an average channel gain to noise ratio (CNR) u k between each antenna and each user into consideration.
  • Equation 1 The CNR (that is, U k ) of channels are represented as Equation 1:
  • Equation 1 M is the number of antennas, N is the number of sub-carriers, and ⁇ k,m,n is CNR when an n th sub-carrier passes through a channel between a k th antenna and an m th antenna.
  • Equations 2 and 3 a symbol error rate with respect to a user k is P e (k), and a signal to noise ratio (SNR) gap is e , the SNR gap (that is, e ) and the CNR (that is, ⁇ k,m,n ) are represented as Equations 2 and 3.
  • H k,n,m is a channel gain coefficient for a k th user, an n th sub-carrier, and an m th antenna
  • ⁇ 2 n,m is noise power for an n th sub-carrier through an m th antenna. Since the transmitter obtains the CNR from each user through a feedback channel in an ASA scheme, the CNR can be obtained for all channels. Therefore, if the number of users is K, an amount of channel information received from all users is K ⁇ M ⁇ N.
  • the sub-carrier to be allocated to each user is determined through the sub-carrier allocation algorithm based on the ⁇ k,m,n obtained through the feedback channel.
  • channel information of each antenna for each sub-carrier frequency is compared and a user signal is transmitted through an antenna having superior channel characteristics and frequency by using antenna selective diversity.
  • the four feedback channels ⁇ 1,n,1, ⁇ 1,n,2, ⁇ 2,n,1, and ⁇ 2,n,2 are formed with respect to n th sub-carrier.
  • the number of sub-carriers n k to be allocated to each user is obtained by using an average of ⁇ k,n,1 and ⁇ k,n,2 corresponding to each user and the sub-carriers are allocated to an antenna representing the superior channel characteristic.
  • the conventional signal antenna system is used for multi-users, only a channel characteristic for a single antenna may exist when allocating the sub-carriers, so a diversity effect cannot be obtained. That is, as the number of antennas increases, the probability of inferior channel characteristic in all channels decreases, so the diversity effect can be improved and an error rate can be reduced. In addition, a probability of null creation can be reduced so that a data transmission rate can be improved.
  • FIG. 2 is a block diagram illustrating a structure of a MIMO OFDMA system for transceiving data by allocating sub-carriers according to a first embodiment of the present invention.
  • data for K users 201 are allocated to predetermined antennas and sub-carriers through a sub-carrier allocator 203 based on channel information per each user.
  • the sub-carriers are transmitted according to allocation information of sub-carriers through Inverse Fast Fourier Transforms (IFFTs) 205 and 213 , guard interval inserters 207 and 215 , parallel to serial converters 209 and 217 and antennas 211 and 219 .
  • IFFTs Inverse Fast Fourier Transforms
  • FIG. 2 illustrates an MIMO OFDM system including m transmission antennas and K users.
  • N sub-carriers are used and an ASA scheme of a closed-loop transmit diversity is employed.
  • a structure of the transmitter shown in FIG. 2 is substantially identical to the structure of the transmitter shown in FIG. 1 .
  • the conventional OFDMA system includes a single antenna structure so the sub-carrier is allocated to each user.
  • the MIMO OFDM system must allocate the sub-carrier to each user and each antenna.
  • the sub-carrier allocation algorithm by means of the sub-carrier allocator according to an embodiment of the present invention will be described later with reference to FIGS. 3 to 5 .
  • each channel has a channel response for N sub-carriers.
  • N information included in each channel is transmitted, so K ⁇ M ⁇ N information is fed back.
  • the structure of the receiver of each user is substantially identical to the structure of the receiver shown in FIG. 1 , except that the receiver shown in FIG. 2 obtains the user signal by demodulating its own sub-carrier based on information related to the sub-carrier allocated by the sub-carrier allocator according to the an embodiment of the present invention.
  • FIG. 3 is a flow chart illustrating a sub-carrier allocation procedure in a MIMO OFDM scheme according to a first embodiment of the present invention.
  • a base station receives channel information related to sub-carriers per each antenna with respect to each user (step 301 ). As shown in Equation 1. an amount of the channel information is K ⁇ M ⁇ N, which corresponds to the number of channel information related to each sub-carrier for each user.
  • the base station determines the number of sub-carriers for each user (step 303 ) and allocates the sub-carriers to users and antennas (step 305 ). At this time, sub-carriers must be allocated to each user by taking channel information of each antenna and each sub-carrier into consideration.
  • the method for allocating the sub-carrier according to the first embodiment of the present invention includes two steps of determining the number of sub-carriers to be allocated to each user and allocating the sub-carriers to each user and each antenna.
  • the number of sub-carriers to be allocated to each user In order to allocate the sub-carriers to each user and each antenna, it is necessary to determine the number of sub-carriers to be allocated to each user.
  • the number n k of sub-carriers to be allocated to each user must be determined by taking the minimum bit rate B min (k) and the maximum transmission energy E max (k) into consideration. If the maximum transmission energy E max (k) is small, a desired bit rate cannot be obtained even if all sub-carriers are allocated to one user. In this case, it is necessary to reduce the bit rate or increase the transmission power.
  • the number of sub-carriers allocated to all users is less than or more than the number of the sub-carriers, it is necessary to adjust the maximum transmission energy or the number of sub-carriers by removing some of the sub-carriers.
  • n k
  • the initial number n k is determined such that the maximum bit number b max per each symbol can be applied to all sub-carriers and total transmission energy E T (k) per each user is determined through the initial number n k of sub-carriers, an average CNR and a minimum bit rate.
  • transmission energy per each user is adjusted by using an extra sub-carriers, thereby increasing the number of sub-carriers to be allocated. That is, when the total transmission energy E T (k) per each user is larger than the maximum transmission energy E max (k), the number n k of sub-carriers is increased one by one so as to maintain the total transmission energy E T (k) below the maximum transmission energy E max (k). If the sum of the sub-carriers allocated to each user is smaller than the total number of the sub-carriers, the maximum transmission energy E max (k) is reduced.
  • new transmission energy E new (k) is defined, in which the number of sub-carriers allocated to each user is reduced by one, thereby reducing the number of sub-carriers while minimizing an interval between the new transmission energy E new (k) and total transmission energy E T (k).
  • the priority includes reference priority and actual priority.
  • the reference priority can be defined as represented in Equation 5 based on the number of sub-carriers allocated to each user in step 1:
  • the actual priority can be defined as represented in Equation 6:
  • a sub-carrier allocation matrix A including antennas for each user and sub-carriers is defined.
  • a sub-carrier component that is, sub-carriers for a corresponding antenna
  • the sub-carriers are compared with each other based on the CNRs of the channels, thereby finding a sub-carrier n having the best CNR.
  • an antenna having a superior channel state is selected to transmit the signal.
  • the actual priority is re-calculated for all users so as to sequentially allocate sub-carriers to users, who have the greatest difference with respect to the reference priority, that is, who have a plurality of sub-carriers and sub-carrier allocation thereto that is delayed.
  • sub-carriers having the best CNR and the sub-carrier allocation matrix A thereof is not “1” are allocated.
  • FIG. 4 is a flow chart illustrating a procedure for determining the number of sub-carriers to be allocated to each user according to a first embodiment of the present invention.
  • the sub-carriers are properly allocated to each user according to feedback channel information based on a minimum transmission bit rate required by each user and maximum transmission energy.
  • step 401 the number of sub-carriers for each user is initialized.
  • a minimum sub-carrier n k is allocated to each user based on a minimum bit rate B min (k) required by each user and the total transmission energy E T (k) for each user is calculated.
  • the sum of sub-carriers allocated to each user is compared with the total number N of sub-carriers (step 403 ). If the sum of sub-carriers allocated to each user is larger than the total number of the sub-carriers, the number of sub-carriers allocated to each user must be reduced. In addition, if the sum of sub-carriers allocated to each user is smaller than the total number of the sub-carriers, the sub-carriers must be further allocated to each user by using extra sub-carriers.
  • steps 403 and 405 If it is determined in steps 403 and 405 that the sum of sub-carriers allocated to each user is identical to the total number of the sub-carriers, it indicates that the sub-carrier allocation for each user has been completed, so the sub-carrier allocation procedure ends.
  • step 403 if it is determined in step 403 that the sum of sub-carriers allocated to each user is smaller than the total number of the sub-carriers, it is necessary to increase the number of the sub-carriers one by one until total transmission energy for each user does not exceed maximum energy allocated to each user. That is, a value of a user k is set to 1 in step 415 , and it is determined whether the number of sub-carriers is increased in step 417 .
  • the total transmission energy E T (k) for the user k is larger than the maximum transmission energy E max (k)
  • the number of sub-carriers allocated to the user k is increased, thereby preventing the total transmission energy E T (k) for the user k from exceeding the maximum transmission energy E max (k).
  • the total transmission energy E T (k) is calculated in relation to the increased sub-carriers in step 423 .
  • step 417 it is again determined whether the sum of sub-carriers allocated to each user exceeds the total number of sub-carriers (step 425 ). If the sum of sub-carriers allocated to each user is smaller than the total number of sub-carriers although the number of sub-carriers allocated to each user is increased through the above procedure, that is, when the extra sub-carriers are added, a maximum energy to be allocated to each user, that is, the maximum transmission energy E max (k) for each user is reduced by a predetermined value in step 427 . After that, the sub-carrier allocation increase procedure is repeatedly carried out with respect to each user.
  • the maximum transmission energy E max (k) for each user is set to a smaller value so as to further allocate the sub-carriers to each user.
  • the maximum transmission energy E max (k) can be continuously reduced by a predetermined value until all sub-carriers have been allocated to each user.
  • step 407 the number of sub-carriers allocated to each user is reduced by one and new transmission energy E new (k) is calculated based on the sub-carriers allocated to each user.
  • step 409 users are aligned in the order of a gap difference between the new transmission energy E new (k) and the total transmission energy E T (k).
  • step 411 the number of sub-carriers for a user having a least gap difference is reduced one by one in step 411 .
  • step 413 the total transmission energy E T (k) becomes an energy value, which is obtained when the number of sub-carriers has been changed.
  • the sub-carriers are allocated to each user and each antenna according to a predetermined procedure shown in FIG. 5 .
  • FIG. 5 is a flow chart illustrating a procedure for allocating sub-carriers and antennas to each user according to a first embodiment of the present invention.
  • a sub-carrier allocation matrix A k,n,m is set to “0” at step 501 .
  • the sub-carrier allocation matrix A indicates a use of the sub-carrier in each antenna per each user k.
  • the reference priority P 0 (k) is calculated based on Equation 5, and a set S 1 of sub-carriers, in which the sub-carriers are aligned in an order of an n k value, is determined, thereby selecting a user having the greatest number of sub-carriers to be allocated thereto.
  • step 503 ⁇ ⁇ 1 k,n,m is obtained by using feedback channel information of a k th user, and a sub-carrier n and an antenna m having a minimum value are searched. Subsequently, a value of n k is reduced by one in step 505 , and A k,n,m corresponding to the sub-carrier and the antenna searched from the sub-carrier allocation matrix A is changed to “1” in step 507 .
  • step 509 it is checked whether allocation for all sub-carriers has been completed in step 509 . If the sub-carrier to be allocated to users still remain, the actual priority is calculated according to Equation 6 in step 511 . Then, in step 513 , a user having the greatest differential value between the actual priority and the reference priority is selected. In addition, after obtaining ⁇ ⁇ 1 k,n,m by using feedback channel information of the selected user, a sub-carrier n and an antenna m having a minimum value are searched in step 515 .
  • n k is reduced by one in step 517 , and A k,n,m corresponding to the sub-carrier and the antenna searched from the sub-carrier allocation matrix A is changed to “1” in step 519 .
  • Such a procedure is repeatedly performed until n k becomes “0”, so all sub-carriers can be allocated to each user and each antenna.
  • a structure of a transmitter in a MIMO OFDMA system will be described with reference to FIG. 6 .
  • Such a group allocation is performed by means of a group allocation controller 611 according to channel information received from receivers 617 and 619 .
  • the group is allocated while taking a channel state of the sub-carrier in each antenna per each user into consideration.
  • the sub-carriers of each group is multiplied by a predetermined phase, and then, the sub-carriers are combined with each other in each group.
  • the signals processed based on group allocation information of the group allocation controller 611 are allocated to a plurality of antennas 613 and 615 by means of an antenna allocator 609 .
  • Equation 7 a transmission signal is represented as Equation 7:
  • T s is a length of a symbol interval
  • d n is a quadrature amplitude modulation (QAM) data symbol
  • g(t) is a rectangular filter.
  • ⁇ f is a frequency interval between sub-carriers and is represented as Equation 8:
  • Equation 9 a transmission signal s (m) (t) transmitted through the m th transmit antenna
  • the sub-carrier allocation is determined according to a sub-carrier allocation algorithm and the sub-carrier allocation procedure is effectively performed in the base station by using feedback channel information, that is, a channel quality indicator (CQI) transmitted to the base station from user equipments.
  • CQI channel quality indicator
  • SIR signal to interference power ratio
  • an n th sub-carrier is allocated to a k th user and the SIR of a channel of an m th antenna is ⁇ k,m,n, a CQI bit is created based on the above SIR value.
  • an average CQI value can be obtained as represented in Equation 10:
  • Equation 11 the average CQI value obtained through Equation 10 can be quantized into Q information as shown in Equation 11:
  • Equation 11 ⁇ Q ⁇ 2 > . . . > ⁇ 1 > ⁇ 0 is a threshold value for classifying a CQI grade and the SIR value is quantized into Q information based on the threshold value. Channel quality is improved as the value of the CQI increases.
  • ⁇ (g) k,m is a channel used for resource allocation with a high SIR
  • the SIR information is estimated per each group in a user equipment and the estimated CQI information is quantized and transmitted to the base station.
  • the base station determines a proper allocation indicator value according to a predetermined sub-carrier allocation algorithm of the present invention.
  • the sub-carrier allocation algorithm is carried out according to two rules.
  • sub-carrier allocation algorithm is periodically carried out according to a channel state.
  • M is the number of transmit antennas
  • N is the number of sub-carriers.
  • k is a user
  • g is a sub-carrier group
  • m is an antenna.
  • R min (k) is a minimum bit rate required by each user equipment
  • B min (k) is a minimum bit rate required for each user equipment.
  • B d (k) is a desired bit rate.
  • the R min (k) and R d (k) are temporary parameters used in the algorithm. Such parameters are updated as one sub-carrier is allocated.
  • the sub-carrier allocation according to the second embodiment of the present invention is determined by taking the minimum bit rate R min (k) required by the user k into consideration.
  • ⁇ (g) k,m which is a CQI value of an m th antenna when a g th sub-carrier is allocated to a k th user, must be considered. That is, the sub-carrier having high ⁇ (g) k,m is allocated.
  • ⁇ ( g ) k , m _ ) must be selected.
  • the base station receives CQI information in relation to sub-carrier groups and antennas per each user in step 701 .
  • g, R min and R d are initialized in step 703 . If a value of g is larger than mod(g+d,G) in step 705 , a value of d is reset to mod(d+v)G in step 707 . Otherwise, a group allocation procedure is performed while increasing a value of g one by one in step 709 . At this time, the value of d is an offset value for allocating the sub-carriers from different positions for the purpose of random resource allocation.
  • step 711 the minimum bit rate R min (k) is checked for all user k in step 711 . If the minimum bit rate R min (k) is larger than “0”, step 713 is performed. In step 713 , the users are realigned in the order of the minimum bit rate R min (k). In step 715 , the CQI value is determined while fixing the values of k and g, thereby determining a maximum value of m and updating the minimum bit rate R min (k). In step 717 , (k,m) for a given g is finally determined and the R d (k) is updated. Thus, the g th sub-carrier group is mapped with the m th antenna for the k user.
  • the R d (k) is a data rate required by each user.
  • step 721 if it is determined in step 711 that the minimum bit rate R min (k) is smaller than “0”, step 721 is carried out.
  • step 721 if a minimum value of the CQI for all k,m is larger than a predetermined threshold value (th), g is fixed in step 719 .
  • (k,m) ensuring maximum CQI with respect to all k,m is allocated for the g.
  • step 721 if the minimum value of the CQI for all k,m is smaller than the predetermined threshold value (th) in step 721 , a value of k ensuring a maximum Rd is determined in step 723 , and values of k and g are fixed in order to determine the value of m having the maximum CQI in step 725 .
  • an efficient allocation procedure can be performed by using sub-carriers (groups) transmitted from plural users and CQI for each antenna.
  • the test is performed through link-level simulation and performance is checked based on bit error rate (BER).
  • BER bit error rate
  • 1 ⁇ 2 convolution coding and QPSK modulation is performed with regard to the user signal, and 512 sub-carriers are utilized.
  • the sub-carrier allocation algorithm test is performed with regard to two users and two transmit antennas while setting a minimum data rate of each user to 300 and a maximum transmission energy to 30 dB.
  • a “conventional” line represents a test result of a conventional OFDM system including one user and one antenna
  • a “proposed” line represents a test result according to an embodiment of the present invention utilizing the sub-carrier allocation algorithm. It is assumed that channel information for all sub-carriers transmitted from each user is accurate.
  • the proposed scheme represents superior performance as compared with the conventional OFDM scheme.
  • BER is 0.01
  • the proposed scheme improves performance by 6 dB as compared with the conventional OFDM scheme. Therefore, when the sub-carrier allocation algorithm is used, the performance of the mobile communication system can be improved due to the transmit antenna diversity effect.
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